U.S. regulators have approved the most expensive medicine ever, for a disorder that destroys a baby’s muscle control and kills nearly all of those with the most common variant of the disease within a couple years.

The treatment is priced at $2.125 million. Out-of-pocket costs for patients will vary based on insurance coverage.

The gene therapy, developed by Novartis, treats a rare condition called spinal muscular atrophy, which strikes about 400 babies born in the U.S. each year.